The present invention relates to processes for preparing complex metal oxides and to complex metal oxides useful as a source of oxygen and sorbent of carbon dioxide in redox hydrogen processes.
A typical redox hydrogen process is a two-step cyclic process for producing hydrogen on a large scale. In the first step, at least one hydrocarbon (e.g., methane) and steam are reacted in the presence of a complex metal oxide and a steam-hydrocarbon reforming catalyst under reaction conditions sufficient to form substantially pure hydrogen gas and a spent complex metal oxide:CH4+ABOx+1.23 H2O=ACO3+3.23 H2+BOx−1.77,where A is a first metal or combination of metals and B is a second metal or combination of metals, and X is typically an integer from about 1 to about 10.
The presence of the complex metal oxide provides an oxidant species that delivers oxygen to the process, and additionally provides the benefit of removing carbon dioxide from the hydrogen gas product stream according to the reactions:ABOn=ABOn−x+x/2 O2 ABOn−x+CO2=ACO3+BOn−x−1 where A, B and X are as indicated above and n is a number that renders the oxide substantially charge neutral.
In the second step of a typical redox hydrogen process, the spent complex metal oxide is regenerated in the presence of air. As illustrated by the following equation, the regeneration step typically proceeds as follows:ACO3+BOx−1.76+0.38 O2 (from air)=ABOx+CO2 
The redox hydrogen process is fully described in U.S. patent application Publication No. 2002/0010220, which is incorporated herein by reference in its entirety.
Although preparation of complex metal oxides is known in the art, improved methods of making the complex metal oxides are sought by those skilled in the art. For example, Inorg. Chem. 23, 1206-1210 (1984) discloses the synthesis of Ca2FeMnO5 from a carbonate precursor in which a solution of nitrate salts of Ca(II), Mn(II), and Fe(II) are precipitated by addition to hot, aqueous NaHCO3 under continuous addition of CO2. The resultant complex carbonate is calcined under an air or O2 purge to yield the desired oxide.
Similarly, U.S. patent application Ser. No. 11/165,720 discloses the preparation of a redox active oxide, Ca2FeMnO5, by adding aqueous solutions of Ca(NO3)2, MnCl2, and Fe(NO3)3 into hot, aqueous NaHCO3 under continuous addition of gaseous CO2 to precipitate complex corbonate followed by calcining the resultant complex carbonate under flowing O2 to yield the oxide. Such processes, however, suffer from drawbacks. For example, the continuous addition of gaseous CO2 during the reaction introduces yet another reagent into the process, which is potentially costly and requires storage of a pressurized gas.
Mater. Lett. 30, 163-167 (1997) discloses the preparation of Ca2FeMnOy from CaCO3, MnCO3, and Fe2O3 by repeated calcinations of pellets of the materials at 1,150° C. to 1,200° C. Characterization of the oxide indicates that y=5.06. The powder X-ray diffraction (PXRD) pattern of the resultant oxide is similar to that of SrMnO2.5 and Ca2Fe2O5 and had a brownmillerite-like structure. Although practical at a small-scale bench operation, the energy requirements for the repeated calcinations make the process inefficient for a large-scale commercial manufacturing of such oxides.
Accordingly, there is a need in the art for a simpler, safer, and more efficient process for making redox active complex metal oxides that will serve as a useful source of oxygen and sorbent of carbon dioxide in redox hydrogen processes.